1. Field of Invention
The Vacuum Freezing Ambient Pressure Melting Process, denoted as the VFAPM Process, is a separation process that is useful in separating solvent from a solution that contains one or more low volatility or non-volatile solutes. It can be used in (a) desalination of sea water and brackish water, (b) water reuse, (c) pollution abatement, (d) concentration of aqueous and non-aqueous industrial solutions such as acid, alkali and salt solutions, (e) separations of solvents from solutions obtained in various extraction operations, (f) concentration of various juices such as orange, apple, tomato, beet and cane juices and (g) conditioning of aqueous and non-aqueous gels such as gelatin and waste water sludge.
A novel low pressure vapor generator and liquefier denoted as a "Sub-Triple Point Vapor Processing Unit" or simply as a "Sub-T.P. Vapor Processing Unit" has also been introduced. A sub-triple point vapor is defined as a vapor or a vapor mixture whose pressure is lower than the triple point pressure of the major component. A Sub-T.P. Vapor Processing Unit generates and liquefies a sub-triple point vapor by subjecting the vapor generated to a desublimation operation or a mixed condensation operation and a desublimate melting operation. Sub-T.P. Vapor Processing Units can be used in (a) various vacuum freezing processes such as the VFAPM Process of the present invention and the VFMPT Process (Vacuum Freezing Multiple Phase Transformation Process) described in U.S. Pat. No. 4,505,728, (b) the Distillative Freezing Process described in U.S. Pat. Nos. 4,378,984, 4,451,273 and 4,578,093 and (c) most freeze-drying processes. The part of the unit used to desublime or mix-condense the sub-triple point vapor and melt the resulting desublimate or solid phase is referred to as a "Sub-Triple Point Vapor Liquefier" or simply as a "Sub-T.P. Vapor Liquefier."
2. Brief Description of the Prior Art
The VFAPM Process is a vacuum freezing process that can be used in the separation of both aqueous and non-aqueous solutions. Many vacuum freezing processes have been introduced by workers in the desalination field. Some of these processes have been tested only in bench scale units and only a few have been tested in pilot plant scale operations. None of these processes has been commercialized and most of these have been considered abandoned. The processes introduced are as follows:
(1) Vacuum Freezing Vapor Compression (VFVC) Process developed by Colt Industries;
(2) Vacuum Freezing Vapor Absorption (VFVA) Process developed by the Carrier Corporation;
(3) Vacuum Freezing Ejector Absorption (VFEA) Process developed by Colt Industries;
(4) Absorption Freezing Vapor Compression (AFVC) Process Developed by Concentration Specialists, Inc.;
(5) Vacuum-Freezing Vapor-Freezing (VFVF) Process invented by Ralph E. Peck;
(6) Vacuum Freezing Multiple Phase Transformation (VFMPT) Process co-invented by Chen-Yen Cheng and Sing-Wang Cheng and under development by Calyxes R & D Corporation;
(7) Vacuum Freezing solid Condensation (VFSC) Process developed by the Catholic University of America;
(8) Vacuum Freezing High-Pressure Ice-Melting (VFPIM) Process introduced by Chen-Yen Cheng and Sing-Wang Cheng.
Referring to the processing of an aqueous solution by any vacuum freezing process, the aqueous solution is introduced into a chamber which is maintained at a pressure that is somewhat lower than the vapor pressure of the solution at the freezing temperature of the solution to thereby simultaneously flash vaporize water and form ice crystals. As the result of this operation, a low pressure water vapor, referred to as a first vapor, and an ice-mother liquor slurry, referred to as a first condensed mass, are formed. In the case of sea water desaltination, this pressure is around 3.5 Torr. The low pressure water vapor formed has to be removed and transformed into a condensed state; the ice crystals have to be separated from the mother liquor and the resulting purified ice has to be melted to yield fresh water. Furthermore, the heat released in transforming the vapor into a condensed state has to be utilized in supplying the heat needed in melting the ice. The processes described utilize different ways of vapor removal, different ways of transforming the vapor into condensed states and different ways of accomplishing the heat reuse.
A sub-triple point vapor is defined as a vapor whose pressure is lower than the triple point pressure of its major component. A subtriple point vapor desublimes to form a solvent solid upon a constant pressure cooling and the desublimation temperature is lower than the normal melting temperature of the solvent solid. When a sub-triple point vapor mixture is cooled, it may undergo a mixed condensation operation by partially condensing into liquid and partially condensing into solid at a temperature that is also lower than the melting temperature of the solvent solid. Therefore, the heat released in the desublimation operation or a mixed condensation operation of a subtriple point vapor or vapor mixture cannot be used in supplying the heat needed in melting a mass of the purified solid of the major component. Conversely, a super-triple point vapor is defined as a vapor whose pressure is higher than the triple point pressure of its major component. A super-triple point vapor condenses to form a solvent liquid upon a constant pressure cooling and the condensing temperature is higher than the normal melting temperature of the solvent solid. Therefore, the heat released in condensing a supertriple point vapor can be used in supplying the heat needed in melting a mass of the purified solid of the major component.
Vacuum freezing processes may be classified into Type A processes and Type B processes. In any Type A vacuum freezing process, a mass of sub-triple point vapor (also referred to as a first vapor) is formed in its vacuum freezing operation described, and a mass of super-triple point vapor (also referred to as a second vapor) is produced in an amount substantially equal to or more than that of the first vapor. The super-triple point vapor produced may be at a near triple point pressure or may be at a pressure substantially higher than the triple point pressure. In each of these processes, the second vapor is generated for one or both of the following purposes:
(a) to melt a mass of purified solvent solid and utilize the heat of condensation of the second vapor, or absorbing solution.
The VFVC, VFVA, VFEA, AFVC, VFVF and VFMPT Processes are Type A processes. However, in Type B vacuum freezing processes, a mass of super-triple point vapor is either not produced at all or the amount produced is only a small fraction of the amount of sub-triple point vapor generated. The VFSC Process and the VFPIM Process of the prior art and the VFAPM Process of the present application are Type B vacuum freezing processes.
The prior art processes are briefly reviewed in the following:
(1) The Vacuum Freezing Vapor Compression (VFVC) Process is a Type A process and is described in the Office of Saline Water, Research And Development Report 295. In this process, the low pressure water vapor (the first vapor) is compressed to a pressure higher than the triple point pressure of water (4.58 Torr) and is then brought in direct contact with purified ice to thereby simultaneously condense the water vapor and melt the ice. The main disadvantage of this process is that the special compressor designed to compress the low pressure water vapor cannot be operated reliably and the compressor efficiency is low. The super-triple point vapor produced is at a near triple point pressure and is used to melt the purified ice, thus reusing the heat of condensation.
(2) The Vacuum Freezing Vapor Absorption (VFVA) Process is also a Type A Process and was developed by the Carrier Corporation up to 1964, but the work has been discontinued. The process is described in the Office of Saline Water, Research and Development Report No. 113. In the process, the low pressure water vapor is absorbed by a concentrated lithium bromide solution. The diluted solution is reconcentrated by evaporation and the water vapor so formed is condensed to become fresh water. Heat of absorption is removed by a recycling water stream through a heat transfer surface; the recycling water stream is then used to melt the ice crystals. In this process, the super-triple point vapor is produced in the course of concentrating the weak absorbing solution and is produced at such a high pressure that it can be condensed by sea water in the condenser.
(3) The Vacuum Freezing Ejector Absorption (VFEA) Process is also a Type A process developed by Colt Industries and is described in the Office of Saline Water, Research and Development Report No. 744. In the process, the low pressure water vapor obtained in the freezing step is compressed by a combination of stem ejector and absorber loop. A concentrated sodium hydroxide solution is used to absorb a part of the low pressure vapor, the diluted sodium hydroxide solution is boiled to form water vapor at 300 Torr and regenerate the concentrated solution. In the ejector, the water vapor at 300 Torr is used to compress the remaining low pressure water vapor to produce a super-triple point vapor. The super-triple point vapor produced is at a near triple point pressure and is used to melt purified ice and reuse the heat of condensation.
(4) The Absorption Freezing Vapor Compression (AFVC) Process is also a Type A process, introduced by Concentration Specialists, Inc., Andover, Mass., and a 25,000 gpd pilot plant has been built at the OWRT (Office of Water Research and Technology) Wrightsville Beach Test Station. The Absorption Freezing Vapor Compression (AFVC) Process is a vacuum freezing process in which the freezing is accomplished in a stirred tank crystallizer due to the evaporation of water vapor which in turn is absorbed in an adjacent chamber by a concentrated solution of sodium chloride (NaCl). The NaCl solution, diluted by the water vapor, is pumped to a generator where it is concentrated to its original strength by a vapor compression cycle using a closed circuit refrigerant as the working fluid. The vapor compression cycle operates between the absorber and generator, taking the heat that is associated with absorption and pumping it up to a level such that it can be used to evaporate the absorbate in the generator. The vapor liberated in the generator is a supertriple point vapor which is used to melt the ice in direct contact. It is noted that the super-triple point vapor is produced at a near triple point pressure in the course of concentrating the weak absorbing solution and is used to melt the purified ice and reuse the heat of condensation.
(5) The Vacuum Freezing Vapor Freezing (VFVF) Process is also a Type A process that was introduced by Ralph E. Peck of the Illinois Institute of Technology and is described in U.S. Pat. No. 3,714,791. In the patent, a batch evaporative desalination method and apparatus having a pair of similar systems for substantially continuous output is described. Each system has three evacuated chambers in vapor communication with each other. In the first chambers, precooled seawater is sprayed for partial vaporization and consequent formation of ice crystals as latent heat is removed from the seawater. Ice crystals are permitted to accumulate in the first chamber and water vapor flows to a second chamber in which refrigeration coils, preferably cooled by cold natural gas, are maintained at a temperature below the triple point so that ice condenses thereon. After a selected interval, spraying of precooled seawater into the first chamber and refrigeration in the second chamber are stopped. Warmer seawater is then sprayed into a third chamber also maintained at low pressure so that a portion of the water vaporizes but without formation of ice in the brine. The water vapor flows to the first and second chambers and condenses on the ice therein to transfer latent heat for melting the ice. Fresh water is withdrawn from the second chamber. Fresh water in the first chamber percolates through the ice crystals for washing and when residual brine is removed, fresh water is withdrawn from this chamber also. In this process, the super-triple point vapor is produced at a near triple point pressure and is used to melt the ice in the first and second chambers and reuse the heat of condensation.
(6) The Vacuum Freezing Multiple Phase Transformation (VFMPT) Process is also a Type A process and is described in U.S. Pat. No. 4,505,728. In the process, a feed containing a volatile solvent and one or more non-volatile solutes is separated to produce a purified solvent product and a concentrate by the following steps: (a) a feed is flash vaporized in a vacuum freezing zone to form a first vapor and a first condensed mass containing solvent crystals and mother liquor, the pressure of the first vapor being lower than the triple point pressure of the solvent; (b) the first condensed mass is separated into a mass of purified solvent crystals and a concentrate in a crystal washing unit; (c) the first vapor is brought to a liquid state in a vapor liquefaction zone comprising several sub-zones by a two-stage transformation involving vapor desublimation and desublimate melting operations; (d) a solvent stream is continuously vaporized in a thin film evaporator within a vapor generation zone to produce a continuous stream of second vapor whose pressure is somewhat higher than the triple point pressure of the solvent; (e) the solvent vapor is brought in contact with the purified solvent crystals to thereby melt the crystals and condense the vapor. The desublimate melting operation may also be accomplished by bringing a part of the second vapor in contact with the desublimate. A unique set of valving means are used to control the flows of the first and second vapors to the vapor liquefaction sub-zones. In this process, the super-triple point vapor is formed at a near triple point and is used to melt the purified solvent crystals and the desublimate.
(7) The Vacuum-Freezing Solid-Condensation (VFSC) Process is a Type B process developed by Professors H. M. Curran and C. P. Howard of the Catholic University of America and is described in the Office of Saline Water, Research and Development Report No. 511. The process is a batch evaporative freezing process in which saline water is sprayed into rotating cylindrical basket at a pressure below the triple point pressure. Continuous removal of vapor results in the formation of an annular ice-brine semi-solid layer on the lateral surface of the basket. The brine is removed by washing and the residual ice is melted. The optimum design requires that the freezing, washing and melting operations be of equal duration. Therefore, the optimum plant consists of three modules, in each of which the freezing, washing and melting operations occur in succession and out-of-phase with the other two. The heat removed by evaporative freezing in one module is used to melt the washed ice in another module. The main mass of washed ice crystals is melted by being brought into contact with the heated surface. In this process, a super-triple point vapor is either not produced at all or is only produced in a small amount.
(8) The Vacuum Freezing High-Pressure Ice-Melting (VFPIM) Process, also a Type B process, is described in U.S. Pat. No. 4,236,382. In the process, an aqueous solution is flash vaporized under a reduced pressure to simultaneously form a sub-triple point water vapor and ice crystals. The ice formed is first purified in a counterwasher and then melted inside of heat conductive conduits under a pressure (e.g. 600 atm.) and the sub-triple point vapor is desublimed to form desublimate (ice) on the outside of the conduits. The latent heat of desublimation released is utilized in supplying the heat needed in the ice-melting operation. The desublimate is removed intermittently by an in-situ dissolution operation utilizing an aqueous solution such as the feed solution or the concentrate; about an equivalent amount of ice is formed inside of the conduits by an exchange freezing operation. The ice so formed is also melted by the high-pressure ice-melting operation described. It is noted that a super-triple point vapor is not produced in this process.
The Distillative Freezing Process described in U.S. Pat. Nos. 4,218,893, 4,378,984 and 4,578,093, is useful in separating a mixture containing at least two volatile components, denoted respectively as A-component and B-component, by simultaneously vaporizing the two components from the mixture under a sufficiently reduced pressure to simultaneously crystallize B-component. The vapor mixture obtained is a sub-triple point vapor mixture and is brought to a condensed state either by a simple condensation to form a liquid mass or a mixed condensation operation to form a condensed mass containing a solid mass and a liquid mass without being substantially pressurized. The process may be conducted to completely eliminate the liquid phase and bring the mixture into the two phase solid-vapor region. Then the solid phase is no longer contaminated by the adhering liquid phase and gives a high purity B-component liquid upon melting. The process is particularly useful in separating mixture containing close boiling components, such as styrene-ethyl benzene mixtures and p-xylene and m-xylene mixtures.
The Sub-Triple Point Vapor Processing Unit contains a two-stage sub-triple point vapor liquefier and can be used in the VFAPM Process, the VFMPT Process, the Distillative Freezing Process, the freeze-drying processes and other processes in which sub-triple point vapors are produced and are to be brought to liquid states.